What causes skin problems in Parkinson’s disease?

Seborrheic dermatitis (SD): The cause of SD in Parkinson’s isn’t clear, but a number of ideas have been proposed:^[1]

• Changes in the skin microbiome
• Impaired immune response to Malassezia spp., a type of yeast found in skin lesions of people with SD.
• Increased amount of fatty acids on the surface of the skin
• Disrupted skin-neurotransmitter signaling
• Impaired skin barrier function caused by genetic factors
• Abnormal shedding of keratinocytes, the most common type of skin cell
• Motor dysfunction: Some researchers have suggested that SD may be indirectly caused by the motor symptoms of Parkinson’s,^[2] since muscle rigidity and the loss of facial muscle control also reduce skin mobility.^[3] In support of this idea, people with moderate to severe levels of Parkinson’s motor symptoms tend to have worse symptoms of SD, or a 1.8-fold increased risk of developing SD if they haven’t developed it yet.^[2] Moreover, treatment with levodopa decreases oil secretion and improves SD symptoms in people with Parkinson’s disease,^[4] further suggesting that SD is related to motor symptoms.

Sweating disorders: Hyperhidrosis (excessive sweating), or hypohidrosis (insufficient sweat production): Sweating disorders in Parkinson’s disease are caused by changes to the autonomic nervous system, which controls sweating.^[5] However, sweating disorders can also be treatment-induced. Therapies that affect dopamine levels, such as levodopa, can cause excessive sweating as a side-effect. Hyperhidrosis can also result when dopamine-based therapies stop working, which can be ameliorated by increasing the dosage.^[6][7] Other treatments that improve motor function, such as deep brain stimulation,^[8] also tend to improve excessive sweating.

Dopamine-affecting treatments for Parkinson’s disease, such as levodopa, can also have negative effects on the skin in certain individuals. Levodopa can cause skin rashes in certain people, although this has been linked to an allergic reaction to a compounding dye used in the drug preparation, rather than a reaction to levodopa itself.^[9][10]